The use of foamed polyvinylchloride (in the following PVC) for the production of many different articles which are obtained, for example, by means of the screw injection molding technique or by means of a transfer or intrusion device or by means of an extrusion process is well known in the art.
Generally, to carry out said techniques a material is used which is based on foamed PVC, said foamed PVC being combined with suitable additives which provide the final product with suitable physical and mechanical properties and which improve its production process.
Among said additives, we can mention, for instance, the plasticizers which have the specific function to soften the starting polymer; the stabilizers which prevent occuring of undesired thermal and/or light degradations; the internal and/or external lubricants which allow the material to be easily worked.
However, it is also known that the products obtained from a material which is based on foamed PVC suitably combined with the above cited additives involve some inconveniences and drawbacks which remarkably limit the use of said products.
First of all, the articles obtained by a material which is based on a traditional foamed plasticized PVC as described above are characterized by a density value which is generally higher than 0.7 g/cm.sup.3, thus said articles are inevitably rather heavy.
Instead, lightness is a very important feature in the production of many different articles, e.g. shoe soles as mentioned above.
A further drawback of the articles belonging to the state of the art and obtained from the above cited material consists in that their external surface generally shows clearly evident streakings caused by the generation of consistent amounts of gazeous products produced by the decomposition of the foaming agent of the starting composition.
It appears to be clear that said streakings are not desirable since they provide the finished product with a poor aesthetical aspect which is absolutely unacceptable for particular kinds of products.
In addition to the above cited streakings, which lower the quality of the article, gazeous bubbles can be formed inside of it. Said Bubbles are uncontrollably generated during the production process and can also have remarkable dimensions.
It is evident that said gazeous bubbles are absolutely undesirable since they can cause swellings and, generally, a decrease of the mechanical properties of the article as well the lowering of the quality thereof.
A further aspect which is very important in specific applications consists in that foamed PVC does not provide the finished product with excellent physical and mechanical properties, e.g. with a satisfactory abrasion resistance.
Owing to the above cited disadvantages and drawbacks caused by the use of a material which is based on foamed PVC, it is often preferable to replace said PVC with other kinds of polymers, e.g. foamed bicomponent polyurethane.
In fact, although foamed bicomponent polyurethane is more expensive as well as more difficult to be molded and treated during the production process than foamed PVC, it allows to produce very light articles having very high superficial quality.